1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer which uses an electrophotographic recording technique or an electrostatic recording technique, and in particular to an image forming apparatus having a cleaning device for cleaning an image bearing member, which bears a toner image thereon, using a blade.
2. Description of the Related Art
In the known image forming apparatuses that employ a structure in which the following processes are repeated: supplying a toner to an electrostatic latent image electrostatically formed on a surface of a photosensitive drum to form a toner image; and transferring this toner image onto a recording material such as paper, it is difficult to shift the entire toner from the photosensitive drum to the recording material in transferring the toner image, and it is unavoidable that a slight amount of the toner remains on the photosensitive drum. Therefore, it is necessary to sufficiently clean out this residual toner every time the toner image is transferred.
Incidentally, various cleaning device have been so far proposed. Among them, there is a cleaning device adapted to scrape off a residual toner by causing a sharp edge of a cleaning blade, which consists of an elastic material such as urethane rubber, to abut against a surface of a photosensitive drum on a further downstream side than a transfer part thereof (downstream side with respect to a travelling direction of the photosensitive drum). This cleaning device has been widely put to practical use conventionally because, for example, it is simple in structure and small, and is also advantageous in terms of costs.
However, in the cleaning device in the image forming apparatus of the related art described above, it is necessary to cause the photosensitive drum to always carry a small amount of toner in its nip (abutment portion) against the cleaning blade in order to cause the cleaning blade to abut against the photosensitive drum steadily and use this toner as a lubricant, to secure a sliding property of the cleaning blade with respect to the photosensitive drum.
If printing is continued in a state in which the toner as the lubricant is not carried on the surface of the photosensitive drum, the abutment of the cleaning blade against the photosensitive drum not only becomes unsteady (chattering or bending is caused), but the edge of the cleaning blade chips. As a result, a large amount of transfer residual toner or the like on the photosensitive drum slips through the cleaning blade, that is, a cleaning failure occurs.
In particular, in recent years, an amount of toner contained in a process cartridge has been increased due to increase in a life of the process cartridge. In order to improve durability of a photosensitive drum following the increase in the amount of toner, a photosensitive drum is used which is not scraped easily when it rubs against a cleaning blade as compared with the conventional one.
In the case in which a hardness of such a photosensitive drum is high, a frequency of occurrence of a harmful effect such as bending of the cleaning blade increases.
In order to eliminate such a harmful effect, for example, a toner image (solid black or lines) for prevention of bending of a cleaning blade is formed in a non-image area on a photosensitive drum, which corresponds to an interval between two pages at the time when images are printed on a plurality of pages continuously, whereby the bending of the cleaning blade is prevented even if it is low in print durability. In this case, a toner is used as a lubricant between the photosensitive drum and the cleaning blade.
However, the toner image for prevention of bending of the cleaning blade has a disadvantage in that an amount of toner consumption increases to cause increase in costs.
Moreover, in an image forming apparatus in which a transfer roller for transferring a toner image onto a recording material from a photosensitive drum is in abutment against the photosensitive drum, there is a harmful effect in that the toner image for prevention of bending of the photosensitive drum adheres to the transfer roller to make contamination of the transfer roller worse.
The present invention has been made in view of the above-mentioned problems, and an object thereof is to provide an image forming apparatus which is capable of suppressing a cleaning failure of an image bearing member.
Another object of the present invention is to provide an image forming apparatus which is capable of suppressing bending of a cleaning blade.
Still another object of the present invention is to provide an image forming apparatus which is capable of suppressing an amount of toner consumption.
Still another object of the present invention is to provide an image forming apparatus including:
an image bearing member;
an electrostatic latent image forming means for forming an electrostatic latent image on the image bearing member;
a developing means for developing the electrostatic latent image in a development position, the developing means developing the electrostatic latent image using a developer containing a toner and an externally added agent having a charged polarity opposite to that of the toner;
a cleaning blade for cleaning the image bearing member; and
a bias controlling means for controlling a bias to be applied to the developing means,
in which the bias controlling means sets a bias applied to the developing means at the time when an interval area passes the development position according to information related to a length of the interval area at the time when electrostatic latent images for a plurality of pages pass the development position.
Other objects of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.